Oscillator tuning system



.A 1945' H. DONLEY OSCILLATOR TUNING SYSTEM Filed May 16, 1942FPEgu/Ewcr Meal/Lawma- 800.905

INVENTOR ATTORNEY Patented Aug. 14, 1945 OSCILLATOR TUNING SYSTEM HughL. Donley, Collingswood, N. 1., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application May 16, 1942, Serial No.443,244

6 Claims.

This invention relates to a frequency modulation system, and inparticular to a method of and means for varying the frequency of anoscillator without causing appreciable amplitude variation.

If the frequency of an oscillator is varied by changing solely one ofthe reactances of the oscillator tank circuit, the impedance of the tankcircuit is altered and the amplitude of oscillation thereby altered inaddition-to the change of frequency. This may be seen from theexpression for the impedance of a tuned circuit; namely,

where r is the effective series resistance of the tank circuit and L andC are its inductance and capacity, respectively. From this expression,it may be seen that if either L or C is varied, the impedance of thetank circuit will vary unless a corresponding change in rr is producedsubstantially in proportion to the change in inductance if inductance Lis varied, or inversely proportional to the change in capacity if thecapacity C is varied.

The present invention is based on an appreciation of these principleswhich are so employed by me that I am able to change the frequency ofthe oscillator without producing any material change in the amplitude ofoscillation. From a broad aspect, I accomplish this result by producingreactance variations in the oscillator tank circuit and simultaneouslytherewith producing variations of effective resistance which tend tomaintain the amplitude of oscillations more constant than in the casewhere only the reactance of the oscillator tank circuit is varied.

The following is a description of the invention accompanied by a drawingwhose single figure illustrates, by way of example only, one embodimentof the invention.

Referring to the drawing in more detail, there is shown one way by whichthe frequency of an oscillator can be varied over a desired rangewithout varying the amplitude of oscillation. The oscillator comprises avacuum tube I having a tank circuit 2 of inductance and capacity coupledbetween the grid and anode. For varying the frequency of the oscillatorthere is provided I acontrol vacuum tube 3 whose grid is coupled to theanode of the oscillator tube through a blocking condenser, as shown, andwhose anode is coupled to a parallel tuned circuit 9 composed of a coil4 and a condenser 5. Tuned circuit 9 is tuned to a frequency above thefrequency of the'oscillator I.

Coil 4 is wound around a ferromagnetic core, preferably magnetite inparticle form, and the ends of this core are positioned in a magneticpath constituted by a U-shaped laminated iron aflair which receives itsexciting flux from a modulator coil 6, as shown. Modulating coil 6 hasits terminals coupled to a low frequency modulating source, such as a.source of speech waves. The lengths of the magnetite core and itssurrounding coil 4 are much longer than their diameters in order toenable as much change as possible in the permeability of the core withchange in exciting flux.

A battery I in series with modulator coil 6 and of suitable valueprovides a direct current bias on the core, thusassuring an operatingpoint on the substantially linear portion of the radio frequencyvolts-permeability characteristic.

The reason that magnetitein powdered or comminuted form is preferred forthe core is because it is an easily saturated material, although othercomminuted ferromagnetic materials may be used.

The anode circuit 4, 5 of the control tube 3 possesses both resistanceand inductive reactance, as a consequence of which theeffectiveadmittance between grid and ground has both a positive susceptancecomponent and a negative conductance component. The former componentaffects chiefly the frequency of oscillation while the latter afiectschiefly its amplitude. I have found that if the anode tuned circuit ofthe control tube l consists of a parallel tuned circuit whose naturalfrequency is above the mean frequency of the oscillator and whosenatural frequency is varied by varying its effective inductance, thatunder suitable conditions of adjustment the variation of the aforesaidnegative conductance is so related to the variation of the aforesaidsusceptance as to tend to maintain the oscillator tank impedancesubstantially constant in magnitude and hence the generated oscillationssubstantially constant in amplitude. The best adjustment of the naturalfrequency of the tuned circuit 4,5 is readily determined experimentallyand will be found to be substantially higher than the oscillatorfrequency. The natural frequency of tuned circuit 4, 5 should not be tooclose to the oscillator frequency, nor too far away from the oscillatorfrequency to materially reduce the frequency shift desired from theoscillator. The experimental procedure is, of course, to observe theamount of amplitude modulation produced as the anode circuit tuning isadjusted and to set the adjustment at the point which gives the leastamplitude modulation.

By applying modulating potentials to modulator coil 6, I am able tochange the permeability of the core as a result of which there is acorresponding change in the inductance of coil 4'. This change inpermeability, in effect, changes t e tuning of the circuit 4, 5 and thisis re- Y I be lower than the Q of the oscillator tank 2.

The Q of circuit 4, I must, of course, be a reasonable one which willnot unduly load the oscillator and which will give a good saturation. Byhaving a reasonably low Q for tuned circuit 4, I, it is possible toobtain a greater percentage frequency shift in the oscillator for thesame power input to the modulating coil compared to a high Q for tunedcircuit 4, 5. By inserting more magnetite material in the core it ispossible to obtain greater permeability but this will reduce the Q ofthe circuit on account of the larger losses in the core.

In one embodiment tried out in practice, the oscillator mean frequencywas fifty megacycles. The control tube used was an RCA 6J5 vacuum tube.The Q of the oscillator tank was 225. The Q of timed circuit 4, Ichanged approximately from 100 to 150 with the core in and out of thecoil 4, the inductance changing by the ratio 1.5 where the core gave aninitial eifective permeability of approximately 2.

Although the illustrated embodiment of the invention has been describedwith particular reference to varying the capacity of the oscillatortank, obviously the invention is not limited to such an arrangementsince, if desired, modifications of the arrangement are readily devisedin which the inductance of the oscillator tank can be varied andsimultaneously therewith the eifective resistance to maintain the ratioof effective inductance to resistance constant. Other changes mayreadily suggest themselves to those skilled in the art without departingfrom the spirit and scope of the invention. For example, the grid of thecontrol tube can be inductively coupled by a small coil to theoscillator tank instead of being capacitively coupled, as shown.

The term angular veloci employed in the appended claims is deemed toinclude both frequency and phase modulation.

What is claimed is:

1. A high frequency oscillator comprising a vacuum tube having an anode,cathode and grid, and an oscillatory circuit coupled between the anodeand grid, a connection from the cathode to a point on said oscillatorycircuit intermediate the ends thereof, means for varying the frequencyof the generated oscillations with a minimum of amplitude variationcomprising a control vacuum tube also having anode, cathode and gridelectrodes, a connection from the grid of said last tube to the anode ofsaid'oscillator tube, a parallel tuned circuit connected between theanode and cathode of said control tube, said tuned circuit having a coilsurrounding a ferro-magnetic core and ,a condenser in shunt to said coiland being tuned to a frequency higher than the frequency of thegenerated oscillations, and means for varying the permeability of saidcore in accordance with low frequency modulating potentials.

2. In combination, an oscillator having a frequency controlling tankcircuit of reactants and eifective resistance, a, vacuum tube having agrid coupled to said oscillator and an anode coupled to a resonantcircuit whosenatural frequency is higher than the'frequency ofoscillations produced by said oscillator, said resonant circuitincluding an inductance coil surrounding a ferromagnetic core, and meansfor varying the per- 7 meability of said core in accordance with 1811 1modulations to thereby reflect variations in admittance upon said tankcircuit.

3. In combination, an oscillator having a frequency controlling tankcircuit of reactance and effective resistance and having a particularvalue of Q, a vacuum tube having a grid coupled to said oscillator andan anode coupled to a resonant circuit of a lower value of Q whosenatural frequency is higher than the frequency of oscillations producedby said oscillator, said resonant circuit including an inductance coilsurrounding a ferromagnetic core, a magnetic path for supplying excitingflux to said core, and means for varying the permeability of said corein accordance with signal modulations to thereby reflect variations inadmittance upon said tank circuit, said means including a modulatingcoil in circuit with said path and coupled to a source of low frequencymodulating energy.

4. A frequency modulation system comprising a high frequencyoscillatonhaving a tuned circutt of reactance and effective resistance,a vacuum tube having a grid coupled to said oscillator and an anodecoupled to a resonant circuit whose natural frequency is higher than thefrequency of oscillations produced by said oscillator, said resonantcircuit including an inductance coil surrounding a ferromagnetic core,and a source of low frequency modulating energy for varying thepermeability of said core.

5. A frequency modulation system comprising a, high frequency oscillatorhaving a frequency controlling tank circuit of reactance and effectiveresistance and having a particular value of Q, a vacuum tube having agrid coupled to said oscillator and an anode coupled to a resonantcircuit of a lower value of Q whose natural frequency is higher than thefrequency of oscillations produced by said oscillator, said resonantcircuit including an inductance coil surrounding a ferromagnetic core,a, magnetic path for supplying exciting flux to said core, and means forvarying the permeability of said core in accordance with signalmodulations to thereby reflect variations in admittance upon said tankcircuit, said means including a modulating coil in circuit with saidpath and coupled to a source of low frequency modulating energy, thelengths of said inductance coil and said ferromagnetic core beingappreciably greater than their diameters.

6. A frequency modulation system comprising a high frequency oscillatorhaving a tuned circuit of reactance and effective resistance, a vacuumtube having a grid coupled to said oscillator, said vacuum tube alsohaving an anode, and means coupling said anode to a circuit tuned to afrequency higher than the frequency of oscillations of said oscillator,said last circuit including a coil wound upon a magnetisable core, andmeans for supplying modulating energy for varying the permeability ofsaid core.

HUGH L. DONLEY.

